1. Field of the Invention
The invention relates to a plasma picture screen provided with a luminescent layer, which comprises a blue, Eu2+-activated phosphor, as well as to a luminescent screen and a phosphor preparation.
2. Description of the Related Art
Plasma picture screens can display color pictures with high resolution, large screen diameter and have a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas and which has electrodes arranged in a grid. The application of a voltage causes a gas discharge which generates light in the ultraviolet range (145 to 185 nm). This light is converted into visible light by phosphors and emitted through the front plate of the glass cell to the viewer.
Phosphors which are particularly efficient when excited by UV are used for plasma picture screens. Frequently used blue-emitting phosphors are, for example, BaMgAl10O17:Eu (BAM) and (Ba,Sr,Ca)5(PO4)3Cl:Eu (SCAP) which are activated by europium in the bivalent state Eu2+. Eu2+-activated phosphors, however, show a strong decrease in the luminous efficacy upon irradiation with vacuum UV light (100 to 200 nm), whereas only little ageing is to be observed upon irradiation with long-wave UV light (200 to 400 nm).
The photodegradation of the Eu2+-activated phosphors when excited by VUV light is caused by the oxidation of the activator Eu2+ to Eu3+. The presence of Eu3+ in the phosphor particles reduces the luminous efficacy, because Eu3+ ions trap the excitons and return to the base state without emitting radiation.
The use of phosphors with activators which are more photostable such as, for example, Ce3+ or Pb2+ could eliminate this problem. These phosphors, however, show a lesser color saturation and a substantially lower luminous efficacy when excited by VUV than Eu2+-activated phosphors.